Hi all,
I'm a Brit, living in Africa, and I have been building new DVB-T/DVB-C transmiter stations for a living mainly for the Pay TV networks.
Now I have gatherd enough equipment to enable me to put up my own DVB-T network in a small town that does not have ANY TV network at all.
Having VERY limited funds I would like to COMBINE two RF modulator signals (64Qam) into ONE transmitter. I have allways (until now) combined the Amplifyers outputs into HUGE combiners and one feed cable !! Can anyone help me with this issue ? I have one modulator output (1W) on 698Mhz 64Qam and the other (1W) on 714Mhz 64Qam
The QUESTION IS : CAN I COMBINE THEM with a spliter/combiner to feed my 250W broadband ?????
Any help would be appreciated ..
David

On paper, you can certainly combine the modulator outputs to feed a common amplifier -- after all, that's exactly how CATV and MATV systems work, although at much lower final power (assuming that the amplifier is capable of handling the total bandwidth of the combined signals). Where you are likely to run into trouble is with filtering the two signals to eliminate out-of-channel interference: a typical digital station has a rather elaborate bandpass (mask) filter between the final amplifier and antenna to enforce the necessary interference requirements without killing off the edges of your own channel. The co-located stations I've seen that share an antenna have their individual mask filters ahead of the combiner, and do not share an amplifier. Of course, there are other factors involved: it's not a trivial thing to design a power amplifier to provide the necessary performance for a DTV signal across a 6MHz channel; accomplishing the same thing across 16 or 18MHz seems unlikely. You also wind up with channel loading issues: the end product will be two channels at less than half the amplifier's nominal capacity. I can't imagine designing a cost-effective and reasonably sized filter to accomplish what you want to do while keeping your signals within technical specs; if you must run two transmitters, it might turn out to be less expensive and simpler in the long run to do this as two separate systems.

But there is another approach: a single carrier can easily handle multiple program streams. For example, you can encode four or five standard definition program feeds, multiplex the encoders' ASI outputs together with PSIP data, and feed that to a single transmitter that runs at the amplifier's full power capacity. The end result would yield a larger coverage area. To a viewer, instead of having two channels as 52-1 and 55-1 (or whatever your channel plan would work those frequencies to), they would see 52-1, 52-2, 52-3, 52-4, and so on.